88 MUSCULAR TISSUE [oil. VI. 



the vesiculae seminales ; (7.) of the uterus and Fallopian tubes ; (8) of 

 blood-vessels and lymphatics ; (9.) of the iris, and ciliary muscle of the 

 eye. This form of tissue also enters largely into the composition (10.) 

 of the tunica dartos, the contraction of which is the principal cause of 

 the wrinkling and contraction of the scrotum on exposure to cold. It 

 occurs also in the skin generally, being found surrounding the secret- 

 ing part of the sweat glands and in small bundles attached to the hair 

 follicles ; it also occurs in the areola of the nipple. It is composed of 

 long, fusiform cells (fig. 114), which vary in length, but are not as a 

 rule more than -g-iy- inch long. Each cell has an oval or rod-shaped 

 nucleus. The cell substance is longitudinally but not transversely 



Fio. 114. Muscular fibre-cells from the muscular coat of intes line highly magnified. Note the longi- 

 tudinal striation, and in the broken fibre the sheath is visible. 



striated. Each cell or fibre, as it may also be termed, has a delicate 

 sheath. The fibres are united by cementing material, which can be 

 stained by silver nitrate. This intercellular substance is bridged 

 across by fine filaments passing from cell to cell 

 1 -1 The nerves in involuntary muscle (both cardiac and plain) do not 

 terminate in end-plates, but by plexuses or networks, which ramify 

 between and around the muscular fibres. 



Development of Muscular Fibres. 



Alljnuscular fibres (except those of the sweat glands which are 

 epiblastic) originate from the mesoblast. The plain fibres are simply 

 elongated cells in which the nucleus becomes rod-shaped. In cardiac 



